In inductors such as choke coils, there is a desire for size and thickness reduction in order to cope with size and weight reduction of electronic apparatuses. For speed and integration increase in LSIs such as CPUs, the inductor is desired for use on large current at several amperes to several tens of amperes in the high frequency region.
Accordingly, there is a desire to inexpensively supply an inductor which is reduced in size and lowered in electric resistance for suppressing heat generation, reduced in loss in high-frequency region and less in inductance value lowering due to direct current superimposition even on large current.
Recently, in DC/DC converters or the like, a circuit scheme called the multi-phase scheme is adopted as a power supply circuit for achieving current increase in the high-frequency band. This circuit scheme is a scheme for sequential operation in parallel by use of a switch while phase-controlling a plurality of DC/DC converters. This scheme has a feature capable of realizing the reduction of ripple currents and increase of current in the high-frequency band with efficiency.
However, the above circuit structure solely is not necessarily sufficient in realizing the increase of current in the high-frequency band. For the choke coil for use on such a power supply circuit apparatus, size reduction and current increase in the high-frequency band is required.
In respect of such a problem, the choke coil disclosed in JP-A-2002-246242 is structured in that in a magnetic material is buried a hollow-cored coil formed by winding in a coil form a conductor wire having an insulation film such as of polyurethane. This magnetic material is made by solidifying magnetic powder whose surface is coated with two kinds or more of resin materials. The magnetic material is fitted with a metal terminal worked by bending. The hollow-cored coil and the metal terminal are electrically connected together by welding, soldering or a conductive adhesive or the like.
However, the conventional choke coil structure requires post-fixing of a metal terminal, making it difficult to reduce direct-current resistance value. Meanwhile, arranging the foregoing coils in plurality corresponding to the number of multi-phases results in an increased setup space, making size reduction difficult. Furthermore, in the case of use in multi-phase, there is a problem that characteristic cannot be fully exhibited because of inductance variation between the plurality of coils.
Meanwhile, when using in the multi-phase scheme a hollow-cored coil formed by winding in a coil form a conductor wire having an insulation film such as of polyurethane, in case a plurality of hollow-cored coils are vertically arranged in line, for example, the total height is increased thus making it impossible to reduce the thickness. Furthermore, such a hollow-cored coil requires to increase the number of turns in order to increase the inductance value, raising a problem of size-increasing of the choke coil itself.